[Haskell-cafe] Tree with collections

[Roberto Zunino]

Henning Thielemann wrote:

> Now I have to write 'show' related code for each collection type. This 
> way I probably duplicate a lot of code that is already written for the 
> Show instances of the collections. To be honest, I use a more special 
> tree structure with even more special "collections" so this may be not 
> really a problem.
>
> Here an instance for a custom list type:
>
>> newtype NewList a = NewList [a]
>
>
>> instance CollShow NewList where
>>    collShow shw (NewList xs) =
>>       "(NewList [" ++
>>          concat (intersperse ", " (map shw xs)) ++ "])"
>
>
> Is there a more straightforward way, preferrably Haskell98?


This avoids duplicating code between Show/ShowColl .

 > instance ShowColl coll => Show (CollTree coll) where
 >    show (CollNode n) = "CollNode " ++ showColl n
 >
 > class ShowColl coll where
 >    showColl :: coll (CollTree coll) -> String
 >
 > instance ShowColl [] where
 >    showColl = show

Also, with GHC extensions and undecidable instances, the following 
incantation seems to work:

 > {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
 > module CollTree where
 > data CollTree coll = CollNode (coll (CollTree coll))
 >
 > instance Show (coll (CollTree coll)) => Show (CollTree coll) where
 >    show (CollNode n) = "CollNode " ++ show n

*CollTree> show (CollNode [CollNode [],CollNode []])
"CollNode [CollNode [],CollNode []]"

However, I can not figure why the typechecker does not loop here (GHC 
6.4.1).

Regards,
Roberto Zunino.